1. A big bubble in the dough, usually found on the crust of a pizza. Often times, this bubble comes out burnt and ruins the appearance and flavor of your pizza.
2. Also a slang term used for an "underground" pimple that can become quite large in diameter and cause redness, similar in appearance to an actual pizza tumor.

1. "Man, this was the best pizza ever! I just wish my slice didn't have that pizza tumor on the crust."
2. John was known all over campus for his terrible acne and disgusting pizza tumors.